As is known, distributed multiple access communication systems include dynamic multi-hop wireless communication systems, which are particularly useful in adhoc networking. In such a system, communication units, very often comprising mobile units, share a common communication channel without a network controller to manage allocation of the common communication channel. In addition, such systems are self-configurable and therefore can be installed quickly where temporary communications is required, such as in emergency operations.
In order to share the common communication channel among the communication units, an efficient communication channel access control protocol, also referred to as a medium access control (MAC) protocol, is required. In addition, the MAC protocol must also address a problem known as “hidden terminal”.
The hidden terminal problem arises when, due to the limited transmission range of the communication units, multiple transmitting communication units within range of a common receiving communication unit may not receive each others concurrent transmissions, and thus, in effect, are “hidden” from one another. Consequently, when the transmitting communication units transmit to the same receiver at approximately the same time, the transmitting units are not aware when their transmissions collide at the receiving communication unit. The hidden terminal problem is known to significantly degrade throughput of the communication system. Further, due to their multi-hop characteristics, a dynamic multi-hop wireless communication system suffers much more from the hidden terminal problem than, for example, a wireless local area network (LANs) system.
A MAC protocol known as Multiple Access Collison Avoidance (MACA) has been used in a dynamic multi-hop wireless communication system, to allow the common communication channel to be shared, and also to alleviate the hidden terminal problem. This has resulted in significant improvement in throughput in the communication system. The MACA protocol implements an exchange of Request-to-Send (RTS) and Clear-to-Send (CTS) messages between a pair of transmitting and receiving communication units, prior to transmission of a data packet. The MACA protocol forms the basis for several more sophisticated protocols. One example is a protocol known as Floor Acquisition Multiple Access with Non-persistent Carrier Sensing (FAMA-NCS), which is substantially immune to the hidden terminal problem and can achieve good throughput.
In wireless LAN, the IEEE 802.11 specification proposes a MAC protocol called Distributed Foundation Wireless Medium Access Control (DFWMAC) for wireless ad hoc LANs. The DFWMAC protocol provides basic and RTS/CTS access method. Here, the RTS/CTS access method comprises a four-way dialog which includes the sequential communication of control packets and data packets, where RTS, CTS and ACK (acknowledgement) packets are control packets. In sequence, the control and data packets are transmitted as follows: RTS-CTS-DATA-ACK between two communication units on the common communication channel. The DFWMAC protocol, however, does not prevent the data packets from colliding with the control packets and/or other data packets. To alleviate the adverse effects of such collisions, the DFWMAC protocol uses a sophisticated modified binary exponential backoff scheme to resolve collisions.
The MACA and DFWMAC protocols are useful in non-realtime applications such as file transfer where the need for communicating time sensitive data packets is minimal. Presently, there is a growing need for such systems to support real time applications such as voice and even video. Consequently, the data packets for realtime applications need to be conveyed more quickly than for example data packets carrying information for file transfer.
In a known proposal for wireless LAN system, to differentiate data packets of realtime applications from data packets that are not, quality of service (QOS) parameters associated with the data packets of realtime applications are used when communicating those data packets. Stations on the LAN with real-time data packets in a transmission queue jam the common communication channel with, what is known as, Black Bursts (BB). The QOS parameters reflect the urgency with which the realtime data packets should be communicated, and duration of BB are determined in accordance with the QOS and is proportional to the delay incurred by the data packets. The station that transmits the BB with the longest duration gets access to the common communication channel, and can then transmit a data packet from its transmission queue. However, this approach fails when hidden terminals exist as those hidden terminals may experience the same delay, and each BB contention period is not guaranteed to result in a unique winner. Thus, real-time data packets will still suffer from collisions if this method were to be used in a dynamic multi-hop wireless communication system.
Another MAC protocol, GAMA (Group Allocation Multiple Access), schedules real-time and non-real-time or so called datagram traffic in a single-hop wireless ad-hoc LAN. The GAMA protocol includes a contention period, during which stations can transmit a request to join a transmission group, and a contention-free period, during which stations in a transmission group take turns to transmit data packets. Again, this approach does not work well if hidden terminals exist. This is because when hidden terminals do not join the transmission group that they may interfere with, then the GAMA protocol cannot ensure that data packets will be free from collision. When hidden terminals do join the transmission group to avoid collision, then all other stations in the LAN have to join the same transmission group one by one. It would be difficult to maintain such a global group in a dynamic multi-hop wireless communication system due to the dynamic nature of the system. Another drawback is the benefit from spatial reuse of communication channels would be limited.
Further, the wireless LAN protocols described above do not support ad hoc routing because in a wireless LAN system a wireless access point can reach all other stations and can relay data packets. In contrast, in a dynamic multi-hop wireless communication system, there is no common access point, hence, routing is another concern.
In addition, unlike in conventional wired networks, a communication unit that acts as a router in a dynamic multi-hop wireless communication system, typically has a single network interface i.e. there are no separate links for the communication unit to route data packets or exchange routing information. This is a particular concern when some communication units in a dynamic multi-hop wireless communication system act as cluster heads or belong to the core of a routing structure. In such circumstances, more traffic will transit through such communication units, in addition to data packets of its own, and therefore, such communication units should have a higher priority in accessing the communication channel to route data packets between other communication units relative to data packets of its own.
Therefore, the LAN protocols discussed above are not directly applicable in a dynamic multi-hop wireless communication system, and although there are existing MAC protocols for dynamic multi-hop wireless communication system, these MAC protocols do not address both the hidden terminals problem and take into account the requirements of communicating time sensitive data packets to support realtime applications.